1. Field of the Invention
This invention is in the field of photocathodes which are responsive to light in the ultraviolet (uv) spectral range to release photoelectrons. The photoelectrons may be amplified or multiplied by conventional devices to provide, for example, a current indicative of a uv light flux, or to produce an image of the uv light source or of an object illuminated with uv light. The present photocathode is based on Al.sub.x Ga.sub.1-x N.
2. Related Technology
A conventional photocathode based on Al.sub.x Ga.sub.1-x N is known in accord with U.S. Pat. No. 4,616,248, issued 7 Oct. 1986 to Messrs. Khan and Schulze. The '248 patent is believed to teach a transmission mode photocathode including a sapphire substrate upon which an Al.sub.x Ga.sub.1-x N active layer is epitaxially grown, either directly or with an intervening buffer layer of Al.sub.y Ga.sub.1-y N, with y&gt;x so that the buffer layer is transparent to the uv light to be detected. The '248 patent teaches that the active layer should be activated at its electron-emitting surface with Cs to provide a negative electron affinity layer.
However, it is believed that the '248 patent cannot provide a transmission mode photocathode with a desirably high sensitivity to uv light because five major requirements for such a photocathode must be met, as follow:
1) The active layer of the photocathode must provide a high crystalline quality for a long diffusion length of photon-excited electrons in the conduction band of the active layer; PA1 2) The active layer of the photocathode must be either P-type or nearly neutral, with a low dopant level, and with effective negative electron affinity to provide conduction band bending adjacent to the surface of the active layer from which electrons are to escape the active layer; PA1 3) The active layer and a physically supporting layer (i.e., a photon-transmitting window supporting the active layer) must form an interface of low crystalline defects to minimize crystal defects in the active layer and recombination of photon-excited electrons back into the crystal lattice at these defects; PA1 4) The active layer must have a high absorption of photons in the spectral region of interest, with a thin active layer having a thickness about equal to the electron diffusion length of this layer; and PA1 5) The supporting layers, including the photon-transmitting window, must have a high spectral transmission in the spectral band of interest.
Considering the disclosure of the '248 patent it is seen that the active layer is epitaxially grown either directly upon the surface of a sapphire substrate, which substrate forms the photon-transmitting support window for the photocathode, or upon an intervening buffer layer of Al.sub.y Ga.sub.1-y N, with y&gt;x. However, without provision to minimize defects in the crystalline lattice at the interface of the sapphire substrate window with the active layer or at the interface of the active layer with the buffer layer, many of the photon-excited electrons will be trapped in lattice defects and cannot yield photoelectron emissions at the emitting surface of the active layer. The intervening buffer layer of Al.sub.y Ga.sub.1-y N taught by the '248 patent will not provide an interface either sufficiently low in or significantly free of crystalline defects.
Additionally, activation of the active layer of Al.sub.x Ga.sub.1-x N with Cs alone is believed to be less advantageous than activation with Cs and O.sub.2 together in providing a more stable negative electron affinity electron-emitting surface of the active layer.